More at 100 femtometers

Main sequence stars get their energy from
the fusion of hydrogen into helium. The pressure and temperature at
the center of stars like our Sun is enough to overcome the electromagnetic
force keeping the bare hydrogen nuclei apart and helium nuclei are formed,
with the release of lots of energy: One helium atom weighs less than
four hydrogen atoms and the difference in mass is converted to energy.
The release of this energy is enough to balance a star's gravitational
contraction, and a star the size of our Sun can maintain this stable
balance for 10 thousand million years or so. Stars more massive than
our Sun can continue the fusion process beyond helium to carbon, nitrogen,
oxygen, and beyond. The largest stars can create all the elements, particularly
in the massive star's final catastrophic explosion as a supernova. The
whole process of stellar evolution is quite well understood and is strongly
dependent upon the mass of the star. It is the strong and weak nuclear
interactions at the level of 1 to 10 femtometers (10-15 to
10-14 meters) that are responsible for the formation of stars
1 million kilometers (109 meters) in diameter and larger.

The primary source of energy in stars like our sun is called the Proton-Proton
Chain. In a three step process, hydrogen is converted to helium:

Step 1

Step 2

Step 3

In order to produce the carbon nucleus, a star must be at least 4/10
as massive as the sun. (A smaller star will not be able to reach the
internal temperatures and pressures required to fuse helium to higher
elements.) The Triple-Alpha Process requires temperatures above 100
million K and very high densities of pure helium. These criteria are
met only after most of the hydrogen core of a star has been converted
to helium by the Proton-Proton Chain.

Triple-Alpha Process

From carbon12, stars as massive as the sun can produce oxygen16
and some neon20 by adding successive helium nuclei. For the
production of heavier atoms, stars more massive than the sun are required.